This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-069044, filed Mar. 13, 2000; and No. 2000-085281, filed Mar. 24, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a discharge plasma generating method, discharge plasma generating apparatus, semiconductor device fabrication method, and semiconductor device fabrication apparatus used in the deposition of thin films of semiconductors such as amorphous silicon, nanocrystalline silicon, polycrystalline silicon, and silicon nitride for use in various electronic devices such as an amorphous Si-based solar cell, a thin-film polycrystalline Si-based solar cell, an LCD (liquid crystal display), a thin-film transistor of a flat panel display, a photosensitive body of a copying machine, and an information recording device, or used in the etching of thin films of semiconductor elements.
The surface processing technologies using a discharge plasma of a reactive gas are being more and more demanded to increase the area and processing speed in the fabrication of diverse electronic devices every year. To meet these needs, research and development are recently being extensively made, not only in the industrial world but also in learned societies, particularly for the use of plasma chemical vapor deposition (to be referred to as PCVD hereinafter) and a very-high-frequency (VHF) power supply having a frequency of 30 to 800 MHz.
Apparatuses and methods of generating a discharge plasma of a reactive gas are disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-325759 (to be referred to as reference 1 hereinafter) and L. Sansonnens et al., xe2x80x9cA voltage uniformity study in large-area reactors for RF plasma depositionxe2x80x9d, Plasma Source Sci. Technol. 6(1997), pp. 170-178 (to be referred to as reference 2 hereinafter).
Unfortunately, the conventional PCVD and plasma etching have the following problems.
(1) The first problem is an increased size of a substrate to be processed. Needs for large-sized solar cells and flat panel display LCDs are increasing. When an amorphous silicon film (to be referred to as an a-Si film hereinafter) is formed on a 50 cmxc3x9750 cm substrate by the conventional PCVD method, the film thickness distribution is, e.g., as shown in FIG. 6A. Also, when an a-Si film is formed on a 100 cmxc3x97100 cm substrate by the conventional method, the film thickness distribution is, e.g., as shown in FIG. 6B. As shown in FIGS. 6A and 6B, as the frequency of a high-frequency power supply increases, the film thickness distribution of an a-Si film increases, so the film thickness uniformity significantly lowers. In the field of LCDS, a film thickness distribution of xc2x15% is permitted. In the field of solar cells, a film thickness distribution of a maximum of xc2x120%, preferably xc2x110% is permitted. Accordingly, when such large-sized substrates are used in the conventional PCVD method, the only practical power-supply frequency is 13.56 MHz, and no VHFs in a frequency band exceeding this frequency are practical.
(2) The second problem is a method of increasing the surface processing speed (improving the productivity) and improving the film quality. To increase the speed of surface processing, the discharge plasma density must be increased. As a method of increasing the discharge plasma density, the use of a VHF in a frequency band exceeding a versatile power-supply frequency of 13.56 MHz is recommended. Also, to improve the film quality, ion damage to a film must be reduced. To this end, reducing the plasma potential related to the ion energy is effective. The use of a VHF which reduces the plasma potential is also recommended for this purpose.
As examples of needs for a high surface processing speed, low cost (a high film formation speed and a large area) and high quality (a low defect density and high crystallinity) are demanded in the formation of thin films for solar cells and thin-film transistors for flat panel displays. Mat. Res. Soc. Symp. Proc. Vol. 424, p. 9, 1997 (to be referred to as reference 3 hereinafter) disclosed a method of increasing the film formation speed and improving the quality of films by using a VHF power supply. A VHF is recently found to be suited to the high-speed, high-quality formation of particularly a thin nanocrystalline Si film which is attracting attention as a new thin film replacing an a-Si film.
When a VHF is used, however, a VHF progressive wave A1 propagating in the forward direction on an electrode and a VHF reflected wave A2 propagating in the opposite direction interfere with each other as shown in FIG. 2A, generating a standing wave A3 as shown in FIG. 2B. This standing wave A3 promotes the spatial nonuniformity of the discharge plasma density, thereby increasing the film thickness distribution of a thin film formed on a substrate 804. This is contrary to the need described in item (1) above.
Other causes which promote the film thickness nonuniformity resulting from this standing wave are as follows.
(a) The first cause is an increase and nonuniformity of the impedance of a propagation path resulting from the skin effect. When a VHF is supplied from a power supply 807 to an electrode 802 via feeding point (feeder distribution center) 809, 809a, and 809b as shown in FIGS. 1A and 1B, the skin effect causes this VHF to propagate, along the surface portion of the electrode 802, as the progressive wave A1 in one direction and the progressive wave A2 in the other direction on the electrode surface as shown in FIG. 2A. This interference generates the standing wave as shown in FIG. 2B. This skin effect appears more notably for a VHF than for a versatile frequency of 13.56 MHz as a high frequency. A surface depth xcex4 at which a current flows by this skin effect is given by
xcex4=(3.14xc3x97fxc2x7xcexcxc2x7"sgr")xe2x88x920.5
where f is the frequency, xcexc is the permeability, and a is the conductivity.
When the electrode 802 is copper, for example, its surface depth is about 19 xcexcm for a frequency of 13.56 MHz as a high frequency, about 10 xcexcm for a VHF of 50 to 60 MHz, and about 5.8 xcexcm for a VHF of 150 MHz. Therefore, in PCVD or plasma etching using a VHF, the impedance of a power-supply propagation path from a power supply to a discharge portion of a discharge electrode increases. This increases the impedance and makes voltage distribution nonuniform. Accordingly, the spatial uniformity of the discharge plasma density cannot be held any longer.
(b) The second cause is mutual interference between a plasma and VHF. When a VHF becomes nonuniform as described above, the plasma distribution becomes nonuniform accordingly, thereby producing a load distribution (the impedance lowers as the plasma density rises) to an electrode. This influences the VHF distribution.
As described above, the discharge plasma density becomes nonuniform owing to the generation of a standing wave by a reflected wave from the electrode end, the influence which the presence of the electrode impedance has on the voltage distribution, and the mutual interference between a plasma and VHF. Consequently, the film thickness uniformity suffers in large-area PCVD using a VHF.
For example, when the dimensions of a parallel plate electrode exceed 30 cmxc3x9730 cm or the VHF frequency exceeds 30 MHz, the influence of the standing wave A3 becomes conspicuous. This makes it difficult to achieve the minimum necessary film thickness uniformity of semiconductor devices.
FIG. 3 is a graph showing a voltage distribution and an ion saturation current distribution by plotting the position (cm) on a discharge electrode on the abscissa and a voltage vpp (V) and ion saturation current on the ordinate. This data is obtained by the conventional method which uses a VHF of 100 MHz. Referring to FIG. 3, a characteristic curve B1 indicates the voltage distribution, and a characteristic curve B2 indicates the ion saturation current distribution. This ion saturation current distribution (B2) is substantially equal to the electron density distribution and can be easily measured. Hence, the ion saturation current distribution is generally used as an index of the discharge plasma density distribution. The voltage distribution (B1) shows that the standing wave A3 is generated on the electrode and the ion saturation current distribution, i.e., the plasma distribution becomes nonuniform accordingly.
On the other hand, in Jpn. Pat. Appln. KOKAI Publication No. 11-111622 (a four-point feeding method: to be referred to as reference 4 hereinafter), the generation of the voltage distribution caused by the standing wave A3 is suppressed by feeding power to four different portions of a ladder electrode. A ladder electrode is formed by assembling a plurality of parallel longitudinal electrode rods 304 and one or more pairs of parallel lateral electrode rods into the form of a lattice. This ladder electrode is assumed to be more uniform than a common parallel plate electrode. In the method of reference 4, the standing wave generation suppressing means as described above is added to this ladder electrode. However, if the electrode dimensions exceed 30 cmxc3x9730 cm or the power-supply frequency exceeds 80 MHz, the effect of preventing the generation of a voltage distribution is substantially lost. This makes it difficult to form a film having a uniform thickness on a large-area substrate.
FIG. 4 is a graph showing voltage distributions by plotting the position (cm) on a ladder electrode on the abscissa and the voltage Vpp (V) on the ordinate. This data shows voltage distributions generated in a certain electrode rod on the ladder electrode when power is fed via four points by the conventional method by using VHFs of 60 and 100 MHz. Referring to FIG. 4, a characteristic curve C1 indicates data obtained when a VHF of 60 MHz is used, and a characteristic curve C2 indicates data obtained when a VHF of 100 MHz is used. Although a relatively uniform voltage distribution is obtained at a frequency of 60 MHz (the characteristic curve C1), a nonuniform voltage distribution is obtained at a frequency of 100 MHz (the characteristic curve C2). Also, in the method of this reference 4, optimum positions of the four feeding points must be found by trial and error, and this requires much labor and time. Furthermore, if the film formation conditions such as the gas pressure and high-frequency power change, the optimum positions of the feeding points also change.
The above problems are also noted in learned societies. Mat. Res. Soc. Symp. Proc. Vol. 377, p. 27, 1995 (to be referred to as reference 5 hereinafter) has proposed a method in which a lossless reactance (coil) is connected to the side away from the feeding side of a parallel plate electrode. In the method of this reference 5, the conditions of reflection of a progressive wave from the electrode end are changed to generate a portion in which the distribution is relatively flat in the waveform of the standing wave A3, e.g., an antinode portion (a portion near the peak of a sine wave) on the electrode, thereby decreasing the voltage distribution in the electrode. However, the method of reference 5 does not do away with the standing wave 3 all together but merely generates a flat portion of a sine wave on the electrode. Therefore, a uniform portion is obtained up to at most about xe2x85x9 the wavelength (0.6 m for 60 MHz, and 0.4 m for 100 MHz). Uniformization within a range exceeding this is in principle impossible. The standing wave A3 shown in FIG. 2B has four nodes and four antinodes. A standing wave like this cannot be uniformized. The method is effective only in a case as indicated by B1 in FIG. 3, in which the length is equal to or smaller than a pair of one node and one antinode.
FIG. 5 is a graph showing a voltage distribution by plotting the position (cm) on a ladder electrode on the abscissa and the voltage Vpp (V) on the ordinate. Referring to FIG. 5, a characteristic curve D indicates data showing a voltage distribution when one end of a parallel plate electrode is terminated by a lossless reactance (coil) at a VHF of 100 MHz. As shown in FIG. 5, the voltage in a region from the end of the electrode to about 30 cm can be made substantially uniform. However, the voltage becomes nonuniform in a region larger than this, so this region cannot be used in film formation.
As described above, when a VHF is used in the conventional PCVD method, it is impossible to generate a uniform discharge plasma in a large area and perform uniform processing if a very large substrate exceeding 1 mxc3x971 m is processed at 60 MHz or if a 30 cmxc3x9730 cm substrate is processed at a frequency exceeding 80 MHz.
M. Noisan, J. Pelletier, ed., xe2x80x9cMicrowave Excited Plasmasxe2x80x9d, Technology, 4, second impression, p. 401, Elsevier Science B.V., 1999 (to be referred to as reference 6 hereinafter) disclosed a technique which supplies two different high-frequency waves to two discharge electrodes. This technique of reference 6 uses one high-frequency wave to generate a discharge plasma and the other high-frequency wave to control the surface bias voltage of a substrate, thereby controlling the amount of active ions flowing into the substrate and the amount of energy incident on the substrate. That is, the purpose of this technique is not to uniformize a plasma when a large-size substrate exceeding 1 mxc3x971 m is processed.
It is an object of the present invention to provide a discharge plasma generating method, discharge plasma generating apparatus, semiconductor device fabrication method, and semiconductor device fabrication apparatus which can generate a uniform discharge plasma in a large area by using a very high frequency (VHF) and can uniformly process a large substrate.
The first subject of the present invention is to eliminate the generation of a standing wave on an electrode in principle and uniformize the voltage distribution on the electrode while a plasma is generated, thereby improving the uniformity (in-plane uniformity) of the film formation rate or etching rate on the surface of a substrate. As described earlier, it is very difficult for the conventional method to make the voltage distribution uniform if the electrode size exceeds xe2x85x9 of a high-frequency wavelength. For example, when a high frequency of 60 MHz is applied to an electrode having dimensions exceeding 1 mxc3x971 m, a standing wave significantly appears to make the voltage distribution extremely nonuniform. Also, when a VHF exceeding 80 MHz is applied to a 30 cmxc3x9730 cm electrode, a standing wave significantly appears to make the voltage distribution very nonuniform.
The plasma density becomes nonuniform at a VHF probably by {circle around (1)} the generation of a standing wave on an electrode, {circle around (2)} the influence of the presence of floating impedance on the voltage distribution, and {circle around (3)} the mutual interference between a plasma and high-frequency wave. The present inventors made extensive studies on these problems and have found that {circle around (1)} the generation of a standing wave on an electrode is the main cause. On the basis of this knowledge, the present inventors have completed the present invention.
The present invention will be simply described below for the sake of easy understanding.
Assume that a phenomenon on two-dimensional electrode is simplified to a phenomenon on one-dimensional electrode and two frequencies are supplied from the two ends of one rod electrode (corresponding to one electrode rod of a ladder electrode). Assume also that the attenuation of the two high-frequency waves is neglected and the reflection at the electrode end is negligibly small. Under the simplified conditions, the high-frequency waves (voltages (xcfx86(V))) supplied from the two ends of the rod electrode are given by
xcfx861=cos(xcfx891txe2x88x92k1z+xcex81)xe2x80x83xe2x80x83(1)
xcfx862=cos(xcfx892txe2x88x92k2z+xcex82)xe2x80x83xe2x80x83(2)
where xcfx89, k, t, z, and xcex8 are the angular frequency (rad/sec), wave number (rad/m), time (sec), position (m), and phase angle (rad), respectively, of each wave.
The wave number k is represented using a phase velocity v (m/sec) and the angular frequency xcfx89 by
k1=xcfx891/v1, k2=xcfx892/v2xe2x80x83xe2x80x83(3)
A high-frequency voltage xcfx86 at each time in each point (portion) on the electrode is given as the sum of equations (1) and (2) by as follows:
xcfx86=xcfx861+xcfx862=cos(xcfx891txe2x88x92k1z+xcex81)+cos(xcfx892txe2x88x92k2z+xcex82)=2cos(xcfx89avetxe2x88x92kmodz+xcex8ave)xc2x7cos(xcfx89modtxe2x88x92kavez+xcex8mod)xe2x80x83xe2x80x83(4)
The individual components in the equation are as follows:
xcfx89ave=(xcfx891+xcfx892)/2, xcfx89mod=(xcfx891xe2x88x92xcfx892)/2,
kave=(k1+k2)/2, kmod=(k1xe2x88x92k2)/2,
xcex8ave=(xcex81+xcex82)/2, xcex8mod=(xcex81xe2x88x92xcex82)/2,
In equation (4) above, the term cos(xcfx89avetxe2x88x92kmodz+xcex8ave) corresponds to a carrier wave, and the term cos(xcfx89modtxe2x88x92kavez+xcex8mod) corresponds to an envelope.
An xe2x80x9cenvelopexe2x80x9d is a curve formed by tracing and jointing the peaks of a modulated high-frequency wave. More specifically, when a high-frequency wave xcfx861 shown in FIG. 7A and a high-frequency wave xcfx862 shown in FIG. 7B are synthesized, a synthetic wave xcfx86 shown in FIG. 7C results. This synthetic wave xcfx86 is represented by equation (4) above, and its component cos(xcfx89modtxe2x88x92kavez+xcex8mod) is the envelope.
First, assume that xcfx891=xcfx892 and xcex8mod is constant with time (xcex8mod(t)=const.), i.e., assume that high-frequency waves having the same frequency and a phase difference which does not change with time are supplied from the two ends. This is equivalent to supplying high-frequency xcfx891 power by dividing it into two portions from a single power supply or to operating a plurality of high-frequency power supplies in synchronism with each other by a high frequency from a single oscillator and supplying their outputs. That is, this case corresponds to the conventional method. When this is the case, the high-frequency voltage xcfx86 on the electrode is represented by
xcfx86=2cos(xcfx891t+xcex8ave)xc2x7cos(xe2x88x92xcfx891/v1xc2x7z+xcex8mod)(5)
This equation (5) shows that a high frequency having a carrier wave cos(xcfx891t+xcex8ave) with the angular frequency xcfx891 and an envelope cos(xe2x88x92xcfx891/v1xc2x7z+xcex8mod) exist on the electrode. The carrier wave dominates a temporal change in the voltage at a certain point, and the envelope dominates the spatial distribution of the voltage. Since xcfx891=xcfx892 and xcex8mod(t)=const, the envelope cos(xe2x88x92xcfx891/v1xc2x7z+xcex8mod) does not change with time and as a consequence becomes a standing wave.
This can be simply explained as follows.
FIG. 2A is a graph for explaining a standing wave generated when the two output high frequencies from a two-output phase-variable high-frequency oscillator are supplied to an electrode while the phase difference between the frequencies is held at a constant value (xcex8mod(t)=const). Referring to FIG. 2A, the waveform of a high frequency A1 indicates a progressive wave of the first power, which is represented by equation (1), and a high frequency A2 indicates a progressive wave of the second power, which is represented by equation (2). FIG. 2B is a graph showing the power distribution of a standing wave by plotting the position on the electrode surface on the abscissa and the square of the envelope term cos(xe2x88x92xcfx891/v1xc2x7z+const.) of equation (5) on the ordinate. When the phase difference between the two output high-frequency power components is fixed to a constant value (xcex8mod(t)=const, e.g., 0xc2x0), these power components are synthesized to form a standing wave as shown in FIG. 2B. This standing wave is the cause of a spatial voltage distribution, i.e., a plasma distribution, and a film deposition distribution in the conventional feeding method.
On the other hand, if the envelope does not stay as a standing wave but moves with time, the generation of a standing wave is suppressed. This is the gist of the present invention. Two examples will be presented below.
(a) When xcfx891xe2x89xa0xcfx892 (a method of supplying high-frequency waves of different frequencies)
When two frequencies are different (xcfx891xe2x89xa0xcfx892 ), a high-frequency voltage xcfx86 at each point at each time is calculated by
From equation (6) above, this high-frequency voltage xcfx86 contains a carrier wave cos(xcfx89avetxe2x88x92kmodz+xcex8ave) having an angular frequency xcfx89ave and an envelope cos(xcfx89modtxe2x88x92kavez+xcex8mod) having an angular frequency xcfx89mod which is generally called xe2x80x9cbeatxe2x80x9d or xe2x80x9chumxe2x80x9d. Since the envelope includes a term xcfx89modt which changes with time, this envelope moves with the passing of time, so no standing wave is generated. This is the first best mode of the present invention.
(b) When xcfx891=xcfx892 and xcex8mod(t)xe2x89xa0const. (a method of supplying high-frequency waves having the same frequency by temporally changing the phase difference)
When frequencies are the same (xcfx891=xcfx892) and the phase difference xcex8mod changes in a split second(at all times) (xcex8mod(t)xe2x89xa0const.), a high-frequency voltage xcfx86 at each point at each time is calculated by
xcfx86=2cos(xcfx891t+xcex8ave)cos(xe2x88x92xcfx891/v1xc2x7z+xcex8mod(t))xe2x80x83xe2x80x83(7)
In equation (7) above, the envelope is cos(xe2x88x92xcfx891/v1xc2x7z+xcex8mod(t)), i.e., includes a term xcex8mod(t) which changes with time. Accordingly, this envelope moves with the passing of time, and no standing wave is generated. This is the second best mode of the present invention.
When the envelope is in a split second (at all times) changed as indicated by items (a) and (b) above, no standing wave is generated on an electrode, so a uniform plasma can be generated.
Note that the conditions of xcfx891xe2x89xa0xcfx892 (the method of supplying high-frequency waves of different frequencies) and xcfx891=xcfx892 and xcex8mod(t)xe2x89xa0const. (the method of supplying high-frequency waves having the same frequency by changing the phase difference) need not be intentionally made. These conditions can be simply realized by the use of two independent high-frequency oscillators. That is, even two high-frequency oscillators set to oscillate the same VHF, e.g., 100 MHz have characteristics unique to the individual oscillators. Hence, actual oscillation frequencies slightly deviate from the set frequency (100 MHz). For example, the first oscillation frequency is 100.001 MHz and the second oscillation frequency is 100.003 MHz, i.e., xcfx891xe2x89xa0xcfx892 holds in a strict sense. Also, frequencies change with temperature variation and the like, so the relationships between frequencies and between phases change at random. Therefore, when a plurality of independent high-frequency oscillators are used, the envelope moves with time, and no standing wave is generated.
Another method of changing the envelope is to change frequency with time (FM modulation: frequency modulation), perform chirp (frequency chirp), or change an amplitude with time (AM modulation: amplitude modulation).
In the formation of a large-area uniform film performed by a plasma by using a high-frequency wave on the basis of this principle, even when the electrode size exceeds xe2x85x9 of the high-frequency wavelength, in which case uniformization is conventionally very difficult, i.e., even for an apparatus for processing a very large substrate exceeding 1 mxc3x971 m at a high frequency of, e.g., 60 MHz, the present invention provides a method of feeding power to a discharge electrode, by which a high frequency is supplied to the electrode such that the envelope function of a high-frequency voltage xcfx86 on the electrode temporally changes, thereby performing high-speed, high-quality film formation as the characteristic of a high-frequency wave and at the same time generating a uniform plasma and forming a uniform film by suppressing the generation of a standing wave on the electrode.
This temporal change of the envelope function is realized by the use of a plurality of high-frequency power supplies.
First, case (a) in which xcfx891xe2x89xa0xcfx892 (the method of supplying high-frequency waves having different frequencies) will be explained.
As practical means of supplying two frequencies (xcfx891xe2x89xa0xcfx892) to an electrode, two independent high-frequency power supplies are used. For example, in two power supplies (containing high-frequency oscillators) having a rated frequency of 60 MHz, each oscillator has an oscillation frequency unique to the oscillator. In practice, therefore, a frequency difference of about a few hundred kHz exists between the two oscillators. This difference between the inherent oscillation frequencies allows xcfx891xe2x89xa0xcfx892 to hold in practice.
Alternatively, to avoid the contingency as described above, a plurality of power supplies having two oscillation frequencies positively made different from each other are used.
If the difference between the frequencies of these high-frequency power supplies is made too large and one frequency largely deviates from an optimum frequency, the properties of film formation and etching significantly lower from the properties of the optimum frequency. To prevent this, the frequency difference is set to 20% or less.
Case (b) described above in which xcfx891=xcfx892 and xcex8mod(t)xe2x89xa0const. (the method of supplying high-frequency waves having the same frequency by temporally changing the phase difference) will be explained below.
In this method, power is fed to a discharge electrode via a plurality of feeding points, and the difference between a phase to one feeder distribution center and at least one phase to another is temporally changed. It is more preferable to use a phase shifter as a means for temporally changing the phase difference. This phase shifter is preferably inserted between an oscillator for determining the oscillation frequency of high-frequency power and an amplifier for amplifying the signal to necessary power.
To suppress a standing wave by using a plurality of power supplies, the one-dimensional modeling mentioned earlier must apply. As one practical condition, a plurality of feeding points of a discharge electrode are desirably arranged in symmetrical positions.
Also, when independent power supplies are used, the difference between VHFs does not stabilize to raise the problem of reproducibility in some cases. Therefore, it is desirable to ensure reproducibility by controlling the VHF difference to a constant value.
Furthermore, an isolator is preferably inserted between an impedance matching circuit and a power supply. This isolator reduces input power to the power supply from another power supply and thereby protects this power supply. The impedance matching circuit matches the impedances of a discharge electrode and the power supply.
The frequency band width of a practical isolator is about 4% for a high-frequency power of 1 kW or less and about 1% for a high-frequency power of about 2 kW. To build a system within this range, therefore, the difference between the frequencies of a plurality of high-frequency power supplies is preferably 4% or less, and more preferably, 1% or less of the average frequency.
Also, in accordance with the magnitude of input high-frequency power to each power supply from the discharge electrode side, the output from another power supply is restricted to reduce input power from this power supply. This prevents a VHF having a different frequency and/or phase from entering the former power supply and thereby prevents damage to the power supply.
To suppress a standing wave by using a plurality of power supplies, high-frequency waves having a plurality of frequencies are supplied from one feeder distribution center. This allows building a system more inexpensively than when the high-frequency waves are supplied from different feeding points. For this purpose, high-frequency power components from a plurality of high-frequency power supplies are mixed by a high-frequency mixer and supplied to a discharge electrode.
As a means for temporally changing the envelope function, different from the method using two frequencies or a temporally changing phase difference, it is possible to temporally change the voltage amplitude of high-frequency power (AM modulation), temporally change the frequency of high-frequency power (FM modulation), or perform frequency chirp.
It is favorable to previously measure at least one of a voltage distribution, plasma generation density distribution, radical generation density distribution, film formation distribution, etching distribution, and semiconductor film characteristic distribution with respect to the difference between a plurality of frequencies, the value of each frequency, the difference between phases, or the range of AM modulation, FM modulation, or frequency chirp, and obtain a necessary uniform distribution on the basis of the measurement result by using time average or time integral by adjusting at least one of the time, period, and frequency of power supply with respect to the difference between the specific frequencies, the value of each frequency, the difference between phases, or the range of AM modulation, FM modulation, or frequency chirp.
Since the envelope distribution of this system changes with time, nodes and antinodes move to turn on/off a plasma. This should not have any influence on the speed and quality of processing by the plasma as an object. Hence, the plasma must keep being ON in a pseudo manner. For this purpose, the cycle of plasma ON/OFF resulting from temporal change in the envelope distribution, i.e., a cycle equivalent to the reciprocal of the difference between the frequencies of a plurality of high-frequency power components, a cycle for changing the phase, or the cycle of AM modulation, FM modulation, or frequency chirp is made shorter than, preferably xc2xd or less the extinction life of an active atom, active molecule, or ion in a discharge plasma. This prevents the discharge plasma from uselessly repeating ON/OFF, so the speed and quality of processing are not influenced.
To keep the plasma ON state in a pseudo manner, the above cycle is determined as follows when a thin silicon film is to be formed by using silane.
Let xcfx84 denote the life of an SiH3 active molecule calculated by
xcfx84≈(xcex94x)2/(2D)xe2x80x83xe2x80x83(8)
where D is a diffusion coefficient (D=2.5xc3x97103 (cm2sxe2x88x921)), and xcex94x is the distance (cm) from an electrode to a substrate.
The cycle described above is made shorter than one or both of the life xcfx84 and the life of 1.1xc3x9710xe2x88x924 sec of a hydrogen atom radical. More specifically, the cycle is preferably xc2xd or less the life xcfx84 or the life of 1.1xc3x9710xe2x88x924 of a hydrogen atom radical.
On the other hand, for an application in which active atoms, active molecules, or ions in a plasma start to be generated in an OFF time after the plasma is generated, e.g., for an application to etching, it is possible to delay the cycle to intentionally form a plasma OFF time to hold an enough OFF time to generate active atoms, active molecules, or ions in the plasma, turn on the next plasma before the active atoms, active molecules, or ions in the former plasma reduce, and turn off the latter plasma, thereby efficiently generating the active atoms, active molecules, or ions. To this end, the cycle of plasma ON/OFF caused by a temporal change in the envelope distribution is made longer than and 10 times or less, preferably 2 to 4 times the generation life of active atoms, active molecules, or ions in a plasma to be generated by the discharge electrode.
Also, the particle generation amount is reduced by setting the cycle to 1 sec or less, preferably 1 msec or less.
Alternatively, the particle generation amount is reduced by making the cycle longer than, preferably twice or more a discharge region dwell time t of a source gas calculated by
t≈(Sxc2x7xcex94x)/Qxe2x80x83xe2x80x83(9)
where S is the substrate area (cm2)
xcex94x is the distance (cm) from the discharge electrode to the substrate
Q is the volume flow rate (cm3/sec)
One desirable practical condition by which the one-dimensional modeling described above simply applies is that the discharge electrode is a ladder electrode or a mesh electrode.
Furthermore, to supply to a substrate electrode high-frequency power which is one of two frequencies for maintaining uniformity and which adjusts the energy of ions incident on the substrate, high-frequency power components are supplied from two or more high-frequency power supplies to each of a discharge electrode having a substrate and a discharge electrode having no substrate.
The discharge electrode preferably has dimensions of 500xc3x97500 mm or more, more preferably in the range of 500xc3x97500 mm to 2,200xc3x972,200 mm, and most preferably in the range of 500xc3x97500 mm to 1,600 to 1,600 mm.
A substrate to be processed can be made of a glass, metal, or resin material. When the substrate is made of glass, the substrate is preferably heated to the temperature of range between 80xc2x0 C. and 350xc2x0 C. in plasma processing. Examples of the glass include soda-lime glass and silica glass. When the substrate is made of a metal, the substrate is preferably heated to the temperature of range between 80xc2x0 C. and 500xc2x0 C. in plasma processing. Examples of the metal include various kinds of stainless steel, aluminum, and an aluminum alloy. When the substrate is made of a resin, the substrate is preferably heated to the temperature of range between 80xc2x0 C. and 200xc2x0 C. in plasma processing. Examples of the resin include polyimide and polyethylene terephthalate (PET).
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.